In surface finishing or conditioning of workpieces, the surface of the workpiece is treated so as to remove irregularities such as machining marks, burrs, parting lines and the like to provide a desired finished surface appearance. This finishing, one variation of which is often referred to as polishing, may involve use of either soft or stiff abrasive members, such as in belt grinding. In this surface finishing technique, however, material is not being removed from the workpiece to provide a specific dimension such as occurs when machining a workpiece. Rather, in surface finishing the workpiece has generally already been machined to the desired tolerances or dimensions, and the surface finishing is for the purpose of providing the desired surface appearance. The surface finishing technique is thus not dimensionally controlled. Rather, the critical aspect of surface finishing is the pressure or force applied between the workpiece and the tool so as to result in the desired surface finish.
In recent years, surface finishing is often carried out using robots which mount either the tool or the workpiece, and various compliant-type follower devices mount the other of the tool and workpiece during the finishing operation. The robot can be pre-programmed and typically mounts the workpiece or part so as to permit manipulated movement thereof through a prescribed path. Such pre-programmed movement, however, cannot readily adjust for changes in contact force or pressure between the tool and workpiece, and thus various types of compliant follower devices have been provided for use in association with the workpiece in an attempt to compensate for workpiece contour and/or position change, while still enabling the tool to follow the workpiece and maintain finishing contact therewith. In its simplest variation, the use of passive compliant follower devices employing springs is well known, although such devices are unsuitable for surface finishing of many workpieces since spring force varies in an approximate linear manner in proportion to displacement, and causes the contact force or pressure between the workpiece and tool to significantly vary.
To improve upon the passive-type compliant or floating follower, such as a mechanical spring device as briefly discussed above, air devices have been developed. The air-actuator follower devices have proven more desirable in that they do possess the capability of maintaining a substantially constant pressure or force on the tool while still enabling the tool to positionally float so as to properly follow the workpiece. The passive air followers, however, are known to be suitable only in environments wherein the response time between the tool and workpiece is relatively slow since fluid systems do not have the ability to provide a rapid response and are unable to properly adjust and follow rapid force changes between the tool and workpiece. Further, these passive systems and particularly the passive air systems are significantly influenced by the inertia of the moving parts as well as by environmental conditions and break-away friction, and are not particularly suitable for low force applications since the inertia effect can cause the actual contact force between the tool and workpiece to be significantly effected when a low constant contact force is the desired objective, and the break-away friction can cause a slow response time so as to prevent the system from timely responding to force changes.
It has been suggested that a passive follower system could utilize constant force electric solenoids so as to permit floating follower movement while maintaining substantially constant force. However, this system was apparently only theoretically suggested, and it is believed that such system is not only undesirable but in actuality is not practical. Electric solenoids are typically designed to have a small predetermined stroke or displacement, and, while solenoids are known which attempt to provide a constant force, nevertheless even these solenoids are known to experience a nonlinear (i.e., a nonconstant) force relationship through the permissible stroke. These solenoids are also nonlinear with respect to output force magnitude relative to magnitude of input current. This thus makes such devices unsuitable for use as a truly constant force follower. Further, with higher force ranges, such solenoids become large and heavy, and have large electrical energy requirements, and hence are unsuitable for these additional reasons.
Because of deficiencies associated with passive follower systems, numerous attempts have been made to develop follower systems which are referred to as "active" systems. The most conventional "active" system is one which is often referred to as an active position system. Such follower systems rely on position control for adjusting contact force or pressure through a feedback system. Most typically an attempt is made to monitor the position of the workpiece and then, through a suitable feedback control system, to adjust the position of the tool in an attempt to maintain constant force. With the active position systems, however, mechanical and circuit processing time delays are generally of sufficient significance as to prevent the force from being adjusted with a sufficiently rapid response time as to maintain such force constant. Further, this arrangement normally requires a rather rigid or stiff drive arrangement, rather than a floating or compliant follower arrangement, and such arrangement does not permit back driving of the follower (i.e., the follower cannot move on its own due to application of external force thereto through the tool).
Because of difficulties associated with active position systems, attempts have been made to provide a floating compliant system which is controlled by an active force system. In an active force system, the follower still possesses compliance so as to be capable of being back driven, but in addition responds to the force between the workpiece and tool and, through appropriate feedback, constantly adjusts the force to compensate for inertial effects and the like as the tool moves back and forth so as to follow the workpiece while still maintaining a theoretically constant contact force. At present, an attempt is being made to provide an active force system employing a pneumatic follower of the type which has been conventionally utilized in passive systems. However, it is believed that such pneumatic-driven active force systems still lack the desired responsiveness, and in addition result in a system having higher mechanical complexity and less reliability such as is inherent in pneumatic control arrangements.
Accordingly, it is an object of this invention to provide an improved floating-type or compliant follower system for use in conjunction with a robotic or automated workpiece finishing apparatus, which follower device in a preferred embodiment cooperates with the tool to maintain a contact pressure or force between the tool or workpiece which can be maintained constant with a high degree of accuracy and precision and with a high responsiveness, so as to overcome many of the disadvantages associated with prior follower systems of the types briefly described above.
More specifically, this invention relates to an improved floating-type or compliant follower device which can be used as a passive device but which can also desirably function as an active force system so as to rapidly respond to both inertial effects and positional variational effects between the tool and workpiece so as to maintain a constant pressure or force between the tool and workpiece to achieve a desired surface finish.
In the improved floating follower device of the present invention, and particularly in a preferred embodiment of this device, there is provided a constant-torque rotatable electric motor, such as a servo motor, wherein the rotatable rotor is subjected to constant torque, which torque remains constant even when the motor is held in a stationary or stall condition by an external load or is driven backwards by the load. The motor output shaft as secured to the rotor is drivingly coupled through a suitable rotary-to-linear drive mechanism to an output member or follower. The motion transfer, in a typical embodiment, may comprise a simple gear-rack mechanism to minimize friction while being capable of being back driven (i.e., driven by the follower). The follower in turn is coupled to a driving tool such as a belt grinder or the like, the latter being maintained in contact with a workpiece which is typically mounted on and is positionally controlled by a robot arm. Energization of the follower motor causes the follower to act against the tool to maintain a constant force or pressure between the tool and workpiece, with the motor rotor being maintained substantially in a stall condition, but being capable of oscillating back and forth while maintaining a substantially constant torque so as to maintain the tool in contact with the workpiece while maintaining a constant contact force or pressure therebetween. The tool can movably respond either toward or away from the center of the workpiece so as maintain proper surface contact therewith while still maintaining constant contact pressure, with the follower suitably reciprocating back and forth so as to follow the workpiece contour, including back driving through the gear-rack mechanism to the rotor when necessary, while maintaining constant torque in the motor and constant contact force at the tool-workpiece contact point.
In the present invention, the follower device can be used as a passive follower as described above, but more desirably is used as an active force follower system. A suitable force sensor is preferably provided so as to accurately and substantially continuously measure the contact force between the tool and workpiece and, through a suitable feedback control loop, the torque of the follower motor can be constantly and substantially continuously adjusted in small increments to maintain the contact force between the tool and workpiece constant, thereby rapidly compensating and adjusting for inertial effects and positional changes of the contact point.
Other objects and purposes of the invention will be apparent upon reading the following specification and inspecting the accompanying drawings.